Electrochemical cells provide electrical power via a chemical reaction. A typical electrochemical cell includes a pair of electrodes called an anode and a cathode separated by an electrolyte composition. The anode, cathode, and electrolyte are contained in a casing and when the anode and cathode are electrically connected to a load, a chemical reaction between the anode, cathode, and electrolyte releases electrons and delivers electrical energy to the load. Although the anode of the invention disclosed herein is generally applicable to electrochemical cells, it was particularly designed for use in metal-air cells.
Metal-air cells utilize oxygen from ambient air as a reactant in an electrochemical reaction to provide a relatively lightweight power supply and include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. Metal-air cells have a relatively high energy density because the cathode utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material, such as a metal oxide or another depolarizable metallic composition. For example, during operation of a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode, reacts with hydroxide ions, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between an anode and the cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air permeable cathode and the anode is electrolytically reformed by reducing to the base metal the metal oxides formed during discharge.
Metal-air cell anodes are made from metals which can be oxidized during discharge in a metal-air cell to produce electrical energy. Such metals include lead, zinc, iron, cadmium, aluminum, and magnesium. Zinc is normally preferred because of the availability, energy density, safety, and relatively low cost of zinc. Typically, anodes include a layer of anodic material which is capable of being oxidized in the cell to produce electricity during discharge of the cell and a current collector made of a metal which is capable of conducting electricity produced during discharge of a cell but not being oxidized during discharge of the cell. Typical metals for the current collector include copper, nickel, tin, titanium, or a metallic substrate coated with these materials.
Metal-air anodes can be made by compressing metal particles, such as metal powder, under suitable conditions of heat or pressure, or both, to form a porous cake. The porous cake can be formed directly onto the current collector. In addition, metal-air anodes can be made from a body of solid, anodic, non-particulate metal, in which case, the current collector is positioned against the body of anodic metal in the cell and is held against the body of anodic material by pressure from the cell casing and the other components of the cell.
Good electrical contact between the anodic material and the current collector of an anode is necessary for efficient production of electricity from the cells. Without good electrical contact between the anodic material and current collector of an anode, the surface of the current collector can become oxidized. The oxidation forms an insulative layer about the current collector. When this insulative layer is formed, the current collector does not effectively conduct electricity from the cell, generates gas within the cell, and causes the cell to fail. This problem is a greater concern with anodes made with a body of solid, non-particulate metal because the current collector is not initially embedded in the anodic material as is the case with anodes made by compressing metal particles to form a porous cake onto the current collector.
Anode current collectors typically are foraminous metal screens, perforated sheet metal or expanded, perforated sheet metal. Anode current collectors are desirably perforated to allow gas produced by the anode to pass through the current collector, and to a vent, for release from the cell. Perforated current collectors are useful in making particulate metal anodes because the anodic layer can be moved as a layer of paste on top of the perforated current collector. The paste settles into the perforations of the current collectors and provides a strong bond between the anodic layer and the current collector. Unfortunately, perforations in a current collector reduce the contact area between the current collector and the anodic layer of an anode and results in uneven current density across the anode. For example, a perforated or foraminous current collector that has forty percent open area, has only a sixty percent current collector contact area.
It is desirable to have full contact between the anodic layer and the current collector and have substantially even current density across the anode.
Substantially full contact between the current collector and the anodic layer can be achieved with an unperforated current collector that extends substantially across an entire side of the anodic layer. Such an unperforated current collector provides substantially even current density across the anode. However, unperforated current collectors are heavy and therefore reduce the overall density of the cell. Unperforated, metal current collectors can also be expensive such as when the current collector is made of silver.
Silver is desirable as an anode current collector because anodes with silver current collectors produce low levels of gas during recharge. To reduce the cost of a silver current collector, copper current collectors are plated with silver. However, the silver on silver coated copper current collectors tends to corrode, thereby exposing the copper which readily produces gas during recharge of the cell. The cost of pure silver sheet metal is expensive and can be cost prohibitive for use as an anode current collector, particularly, in metal-air cells.
Therefore, there is a need for an anode with enhanced reliability, and in particular, an anode that has an effective and durable electrical connection between the anodic material and the current collector, produces a low level of gas, and is economical.